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Per the Wikipedia article for High-test peroxide:

Hydrogen peroxide becomes more stable with higher peroxide content. For example, 98% hydrogen peroxide is more stable than 70% hydrogen peroxide. Water acts as a contaminant, and the higher the water concentration the less stable the peroxide is.

The article then goes on to list various uses of high-test peroxide, most of which are in aerospace applications, either as a monopropellant or as an oxidizer in bipropellants. Most of the hydrogen peroxides used in these applications are around 80%. I would have thought this was due to higher concentrations being impractical due to safety or handling concerns, but per the same page, they're actually more stable. If this is the case, why not just take the performance advantage and use 98-100% everywhere?

Is 98+% hydrogen peroxide especially expensive or difficult to manufacture? Is there some other concern (storage?) that limits practical usability to the more dangerous, worse-performing 80-85% concentrations?

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    $\begingroup$ It may be that pure produces too much heat on its own. With dilute, the water content turns into steam, adding thrust as well as absorbing some of the heat when the peroxide breaks down. $\endgroup$ – Robert DiGiovanni Mar 22 at 23:57
  • $\begingroup$ Robert DG is probably on the right track - you want both energy and expellable mass for best performance. $\endgroup$ – Carl Witthoft Mar 23 at 12:34
  • $\begingroup$ Maybe, but I've run this through ProPEP, and, as with most situations where there's water in the propellant, the specific impulse of 100% is better. For example, with RP-1, ProPEP gives an optimal Isp* (vacuum Isp with an expansion ratio of 1) of 205s at 1 part RP1 to 7 parts 100% H2O2 by mass. For RP-1 with 90%, the optimal Isp* is 203s, with the optimum at the same ratio. Sure, considering a sizable amount of the propellant is water, it's surprisingly close, but it's not better. The exhaust product molecular weight is identical (21.813), so I don't think the water helps thrust much either. $\endgroup$ – DodoDude700 Mar 23 at 12:48
  • $\begingroup$ 100% H2O2 is better performing, there is no doubt in that, more oxygen, more fuel can be combusted and more energy will be released. Water is also produced during RP-1 combustion, more RP-1 combusted - more water vapors produced, so adding water to H2O2 will not greatly affect molar gas mass. Also water has high vaporization energy so it significantly decreases decomposition temperature. Bellow 67% H2O2 will not completely decompose, water will remain as liquid. There are other reasons why concentrations are usually in 80-90% range. $\endgroup$ – WOW 6EQUJ5 Apr 6 at 18:50
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There are several reasons:

  1. Catalyst - In most applications HTP is firstly decomposed to steam an oxygen and than introduced to combustion chamber where spontaneously ignites fuel (usually RP-1). Catalyst pack is usually made from silver plated stainless steel meshes. Many kinds of catalyst can decompose HTP (permanganates, manganese dioxide, lead oxide etc.) but its found that silver is the most effective. Silver can be used up to 92% HTP concentrations, because adiabatic flame temperature in higher concentrations is close to silver melting point and catalyst can be damaged. For higher concentrations than 92% platinum should be used. Platinum is more expensive, more dense and less effective catalyst. Therefore you would lose effectiveness in decomposition, decrease engine trust/weight ratio and increase expenses. Subject of many researches is to substitute platinum with ceramic materials, but ceramic is prone to cracks in severe thermal and pressure gradients. In jet packs it's already substituted but there is no concern that any flying ceramic parts will block injectors or damage turbine blades.
  2. Freezing point - Water is used as freezing point depressant. Pure hydrogen peroxide freezes at −0.43 °C. 98% HTP freezes at -2,5ºC, 90% at -11ºC, 70% at -39ºC etc. enter image description here Stainless steel and aluminum are material compatible with HTP and therefore mainly used for propellant tanks. Both materials have high thermal conductivity and rocket has to pass through stratosphere where temperatures can reach -51ºC so its desirable freezing point to be as low as possible. Usually concentration is a trade off between performance and freezing point. It's found that beyond 50% concentrations have remarkably tendency for super-cooling.
  3. Detonation hazard - Wikipedia considers more stable as less deteriorating. Any concentration of hydrogen peroxide will decompose during storage, pure being the most stable in this terms. But pure hydrogen peroxide is capable of detonating. Although it's difficult to achieve proper detonation, it's still possible and represents risk especially in rocketry where large amounts are stored at once. Few percents of water largely mitigate this risk and it's believed that bellow 90% its no longer capable of detonating if solution is fuel free. Hydrogen peroxide vapors also can detonate. It's found that in 90% solutions water and hydrogen peroxide vapors are not explosive under normal storage temperatures.
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